1. Field of the Invention
The present invention relates to an imaging device, an image reading device, an image forming apparatus, and a method of driving an imaging device.
2. Description of the Related Art
An image reading device illuminates an original with light from a light source, such as an LED, and, with a photoelectric conversion element that is a CMOS image sensor, etc., receives the reflected light and performs photoelectric conversion, thereby generating an imaging signal that is an analog signal. LED is an abbreviation of “Light Emitting Diode” and CMOS is an abbreviation of “Complementary Metal-Oxide Semiconductor”. The image reading device digitizes the imaging signal by performing A/D conversion with an A/D converter, thereby generating imaging data.
When reading a color image at high speed, the image reading device uses a white light source as a light source and receives reflected light with corresponding photoelectric conversion elements via color filters of respective color components, such as R (red), G (green) and B (Blue). In this manner, the image reading device generates imaging signals of the respective color components with the respective photoelectric conversion elements and combines the imaging data of the respective color components after A/D conversion, thereby generating a color image.
When photoelectric conversion is performed for each color component, there is variation in sensitivity to the color components. This results from the use of a phosphor-based white LED where light of a blue LED and light from a fluorescent coating are mixed and from the fact that the color filters do not have ideal spectral characteristics. During manufacturing, variation is caused in the amount of the fluorescent coating of the light source to be applied and the film thickness of the color filters. Thus, the variation in sensitivity to the color components is different from one individual to another. The variation in sensitivity between color components causes a difference in used dynamic range of the A/D converter between color components, which causes a difference in image tone.
For a method of adjusting the sensitivity to each color component, is known a method of amplifying an imaging signal of before A/D conversion with an amplifier where the gain is adjusted for each color component so as to make the best use of the dynamic range of the A/D converter. However, with this method, noise components are amplified as well with the gain of the amplifier and thus the noise level differs between color components, which degrades the image quality.
For a method of adjusting the level of an imaging signal of each color component without adjusting the gain of an amplifier, is known a technology for adjusting the electric charging time according to the sensitivity ratio between color components during exposure of one frame with an area sensor or during exposure of one line with a line sensor.
Japanese Patent No. 4004833 discloses a method of driving a solid-state imaging element, which is a method aimed at adjusting the level of an imaging signal of each color component without adjusting the gain of an amplifier. This method of driving a solid-state imaging element performs an electronic shutter operation intermittently for plural times during the period of performing exposure once. Furthermore, electricity is stored selectively in a reading period correlative to the sensitivity ratio and an image signal is generated according to the stored electricity when the exposure period ends.
As for the conventional technology for adjusting the electric charging time for each color component, if there is a large difference in sensitivity to color components, the time of electric charging for a color to which the sensitivity is high in one line is shortened, which leads to uneven electric charging periods in one line. For example, a device that reads an original by scanning with a carriage, such as a scanner, has a problem in that, when the electric charging periods are uneven in one line, a fine line in the main-scanning direction (a fine line along the direction in which photoelectric conversion elements of one line are arrayed) disappears (cannot be read).
The method of driving a solid-state imaging element disclosed in Japanese Patent No. 4004833 has the same problem, i.e., the number of times of charging is small for a color component to which the sensitivity is high, which leads to uneven electric charging periods in one line. This causes an inconvenience in that, when an original is read by scanning with a carriage, the fine line in the main scanning direction disappears as described above.
In view of the abode-described inconvenience, there is a need to provide an imaging device, an image reading device, an image forming apparatus, and a method of driving an imaging device with each of which an original can be read with high reproducibility.